Oxidation of alkanes is an important reaction for conversion of saturated hydrocarbons to their corresponding alcohols/ketones, which have major applications as plasticizers, solvents, and also as detergent grade alcohols in the production of biodegradable surfactants.
The use of palladium metal and palladium organometallic complexes for alkane oxidation are well known. A majority of literature reports on palladium catalyzed oxidation deals with lower alkane oxidation ranging from methane to butane using molecular oxygen, in a highly acidic medium. Reference is made to the paper, Metallokompleksnyi Katal., 116–29, 1977 by Rudakov et al. wherein the oxidation of saturated hydrocarbons is reported using palladium(II) complexes as catalysts in highly acidic media like sulfuric acid, sulfuric acid-aluminum sulfate, and phosphoric acid-boron trifluoride. The disadvantage of this system is the essential requirement of a highly acidic medium, to conduct the reaction, which is avoided in the present new process. The present invention can be conducted without any solvent, and hence is devoid of any acidic/corrosive components.
Reference is be made to a paper Struktura, Reaktsion. Sposobnost Organ Soedin i Mekhanizmy Reaktsii, Kiev, 69–101 From: Ref Zh., Khim. 1981, Abstr. No. 11B1095, 1980, to Rudakov et al., wherein the oxidation of linear alkane is carried out using palladium sulfate, and nitronium ions in a 80–100% sulfuric acid medium. The oxidant used in this system is nitronium ion. The present invention employs molecular oxygen as an oxidant.
Reference is made to another paper, New J. Chem., 13(10–11), 761–6, 1989, by Herron et al., wherein zeolite supported Fe/Pd bimetallic catalysts are used for the selective oxidation of alkanes at room temperature. Here a mixture of hydrogen and oxygen, or H2O2 is used as an oxidant. In contrast, the present invention employs molecular oxygen as the oxidant in the presence of a Pd(II) complex. No hydrogen or other sacrificial reductant is required.
Reference is also made to a publication in, J. Am. Chem. Soc., 1114(18), 7307–8, by Lin et al., wherein Pd metal is used to catalyze methane oxidation by molecular oxygen in aqueous medium at 70°–110° to HCO2H. Ethane oxidation to acetic acid and formic acid is also observed in the presence of the same catalyst. In this report, the catalyst used is palladium metal in an aqueous medium, whereas, in the present invention the catalyst used is necessarily a Pd(II) complex. In addition in the present invention, solvent is not an essential requirement as the reaction can be conducted with pure substrate.
Reference is made to the paper in, J. Am Chem. Soc., 119(26), 6048–6053, 1997, by Lin et al., wherein a bimetallic Palladium and Copper chloride catalyst is used for low-temperature oxidation of methane, ethane, and butane to corresponding acids and alcohols in the presence of dioxygen and carbon monoxide. The presence of Copper is essential to reoxidize Palladium. The reaction medium used is strongly acidic—aqueous trifluoroacetic acid. In contrast, the present invention employs Pd(II) complex catalyst in a reaction medium devoid of any acid, and does not require any co-catalyst.
U.S. Pat. No. 5,623,090 to Masatake et al., discloses the use of gold particles deposited on titanium dioxide carrier as an oxidation catalyst. This catalyst is reported to catalyze the oxidation of saturated hydrocarbons to alcohols and ketones in the presence of molecular oxygen. In the present invention, the catalyst used is a Pd(II) complex catalyst, and not Au/TiO2.
U.S. Pat. No. 5,235,117, to Jacques et al., reports the preparation of boric acid and its use in the oxidation of saturated hydrocarbons to alcohols. In the present invention the oxidation catalyst used is Pd(II) complex catalyst which catalyses the alkane oxidation in the presence of molecular oxygen No boric acid is employed in the present invention.
Palladium(II) acetate is used to effect the trifluoroacetoxylation of alkanes such as adamantane and methane as well as, arenes in trifluoroacetic acid, as reported in New J. Chem., 13(10–11), 755–60, 1989, to Sen et al., The reaction is made catalytic in Pd(IT) by the addition of a cooxidant, K2S2O8, which reoxidizes the Pd0 formed at the end of the trifluoroacetoxylation step. In the present invention Pd(II) metal complex is used as a catalyst and it does not require any cooxidant to carry out the oxidation reaction.
PCT Publication WO 9214738 A1, to Periana et al, wherein PdSO4.2 H2O is used as a catalyst to convert methane to MeOSO3H in 20% oleum at 100° C. In this report use of PdSO4 as an oxidation catalyst for oxidation of methane to esters and alcohols in highly acidic medium is also reported. In present invention, the catalyst used is Pd(II) complex and the reaction is carried out in acid free solvent and with a pure substrate.
Supported palladium metal is also used for the oxidation of alkanes as reported by Haack et al in Catal. Lett., 34(1,2), 31–40, 1995, wherein Pd/□-alumina and Pd foil is used for methane oxidation, Appl Catal., B, 9(1–4), 251–266, 1996, by Maillet et al., wherein Pd/Al2O3 catalyst is reported for the oxidation of propane in exhaust gas under steam and oxygen/steam system, and in yet another paper by Muto et al., in Catal Today, 35(1–2), 145–151, 1997, wherein alumina coated with a monolayer of palladium loaded silica is reported for oxidation of methane. In the paper by Mazza et al., in Riv. Combust., 50(11–12), 439–443, 1996, palladium loaded titania catalyst is used for oxidation of linear paraffin, n-hexane. (PdCl2-heteropoly acid)/SiO2 system is used for the oxidation of n-C4H10, n-C5H12 at 300° C. in Kinet. Catal. (Transl of Kinet. Katal.), 36 (3), 373–6, 995 by Volkova et al., where as in the present invention the catalyst used is not Pd in metallic state and is also not supported on any inert support. The present invention uses a non supported Pd(II) metal complex.
From the prior art it is clear that the studies relate mainly to oxidation of lower alkanes/cycloalkanes using heterogeneous, supported Pd catalysts, in the presence of a co-catalysts or co-oxidants or in the presence of a reductant like H2 or CO.
These drawbacks are obviated in the present invention, which employs an organometallic Pd complex catalyst, using molecular oxygen as the reactant, in the absence of any co-catalyst or any acidic solvent. In fact the present invention can also be practiced in the absence of any solvent.